Do wheel skirts really work?

[omgwtfbyobbq]

Originally Posted by theclencher

Intuitively skirts make a lot of sense for lowering Cd. Reality seems a bit different though. I did a skirt/grille block/mirror fold/wiper remove college paper on aero and failed to find A-B differences in coast down tests. Then I did practically the same thing again 23 years later on the Lambo and still have realized no results- although with the coming of warm weather and comparable conditions I'm still hoping to find something.

Also I said "dubious" because some of the most aerodynamic cars in the world have no skirts!!! Why, when they are shooting for ultimate aero and ultimate FE bragging rights, would they omit something so important? I think the answer is, skirts really aren't that important:

They aren't that important compared to under/cooling/rear end flow, but it's still something imo. The trade off of a count or two of drag just isn't worth alienating buyers in the eyes of anyone who designs cars. As for being able to tell from coast down tests, from my back of the envelope *guestimations, a ~mph change in relative speed at ~50mph will result in about the same impact on a coast down test than a reduction of drag from .3 to .29 will. Which implies to me that small items like mirrors, windows up, wheel skirts, etc... will be extremely hard if not impossible to test using a coast down. There's going to some speedometer error, timing error, a likely difference in wind speed. The coast down test would need to be from such a high speed, that it would be needlessly dangerous, if not impossible.

*Going from ~49mph to ~47mph at standard everything is analogous to going from ~22m/s to ~21m/s. In a car with A=~2m^2, and Cd=.3, a drop of 1m/s results in 378N compared to 415N. A reduction in drag of a count, so Cd=.29, results in ~403N of force, which means that a 2mph speed error results in 2.5 the difference that a 1 count drop in drag does. Or that a 1mph speed error at ~50mph will have the same impact of wheels skirts, or a mirror delete, or rolled up windows. For the DIY'er, it's pretty much impossible to work around that level of noise.

[CoyoteX]

The only consistent coast down test I could ever get to work right was to coast down a hill where the car reached terminal velocity. When I experimented with the aero mods I did to my car they all increased my terminal velocity and I could repeat it as many times as I wanted going back and forth with the mods. I could start at the top of the hill within a minimum speed and at the same point on the road I was always at the same speed. I could start at the top of the hill faster than the terminal velocity and the car would still slow down to that speed.

The wheel skirts picked up about 1-2mph on my terminal velocity. Around the same amount as the passenger mirror if I remember it right. So they don't make a huge difference but they do help some. Swapping front bumpers made a much bigger difference, almost 5mph. Rear swift wing on the hatch gave me just a tiny improvement. The needle would sit on the other side of of the mark so maybe .5mph

With all my aero mods I had a hard time reaching terminal velocity at the same point, the car was still accelerating and I was not able to get more speed at the top really due to having to climb the other side of the hill. I need to add nitrous or something to get more speed at the top of the hill

The other big hill I had to go over I could coast down and would always hit 55mph at the same spot on the road. I could change something and the car would vary from that spot where it hit 55mph and I could tell if it was better or worse. But that method was nowhere near as consistent as the first hill where I had a long downhill run.

[Bill in Houston]

Originally Posted by omgwtfbyobbq

*Going from ~49mph to ~47mph at standard everything is analogous to going from ~22m/s to ~21m/s. In a car with A=~2m^2, and Cd=.3, a drop of 1m/s results in 378N compared to 415N. A reduction in drag of a count, so Cd=.29, results in ~403N of force, which means that a 2mph speed error results in 2.5 the difference that a 1 count drop in drag does. Or that a 1mph speed error at ~50mph will have the same impact of wheels skirts, or a mirror delete, or rolled up windows. For the DIY'er, it's pretty much impossible to work around that level of noise.

I think that we are talking about two seperate flavors of test:
1- Coast-down: The car is accelerated up to, say, 62 mph. Car is placed in neutral. Time intervals are recorded as the car goes through 60 mph, 55 mph, 50 mph, 45 mph. The times are put into a calculator and a combined Cd and drivetrain/rolling drag are spit out.
2- Terminal velocity: The car is driven down a hill with a consistent grade, and the terminal velocity is measured with the car in neutral. Since the effect is small, repeated trials and strong downhill acceleration (before putting the car in neutral) may be required.

I have not done the math on the coast-down test to see if it has the possibility for better resolution, but I know that one problem with the terminal velocity test is that a 3% reduction in Cd will only change the Vt by 1%, which could be pretty hard to measure, as byobbq was saying. Also, it's just plain hard to know what the Vt is, since you can be within an mph or two of it, and feel like you are topped out. Best to approach from above and below to try to converge on it.

As you guys know, both of these tests are going to be sensitive to temperature and crosswinds, as well as changes in rolling resistance.

Lots of fun.

[rh77]

I'm constantly complaining, with the crosswinds, that I need a rudder.

I've got it -- 4-wheel steering, and you aim the front of the car into the wind, and drive "straight". Gusts, however, would be a problem

RH77

[trebuchet03]

Originally Posted by rh77

I'm constantly complaining, with the crosswinds, that I need a rudder.

I've got it -- 4-wheel steering, and you aim the front of the car into the wind, and drive "straight". Gusts, however, would be a problem

RH77

I think this fiat can do that :P

[rh77]

Originally Posted by trebuchet03

I think this fiat can do that :P

Perfect! Where can I get one?

[trebuchet03]

Originally Posted by rh77

Perfect! Where can I get one?

Poland I'm told :P Only a limited number manufactured.... 1

[rh77]

Originally Posted by theclencher

Just have castering rear wheels and toe brakes for each front wheel... and be an alert driver!

I think you just found the next competitive motorsport! It's got to be harder than drifting

[The Toecutter]

If you make it aero, they may have an effect. You just have to find a way to keep attached flow, perhaps.

The Sidekick would probably have the same problem, but when I get the chance, I'm definately going to try. I just need a way to accurately measure FE, and need to wait for my parents to dump that POS into my hands. I have lots of ideas on how to cut the drag, but I'm thinking more radical **** akin to what basjoos has done.

[red91sit]

Originally Posted by The Toecutter

If you make it aero, they may have an effect. You just have to find a way to keep attached flow, perhaps.

The Sidekick would probably have the same problem, but when I get the chance, I'm definately going to try. I just need a way to accurately measure FE, and need to wait for my parents to dump that POS into my hands. I have lots of ideas on how to cut the drag, but I'm thinking more radical **** akin to what basjoos has done.

I like your ideas :-) BasJoo's car is GREAT. I'd love to see more people doing things like this. I can't make any permentant modifications to my turd, because I REALLY need to sell it and get something smaller and more efficient